Conventional steam crackers are known as an effective tool for cracking high-quality feedstocks that contain a large fraction of volatile hydrocarbons, such as ethane, gas oil, and naphtha. Similarly, regenerative pyrolysis reactors are also known and conventionally used for converting or cracking and to execute cyclic, high temperature chemistry such as those reactions that may be performed at temperatures higher than can suitably be performed in conventional steam crackers. Regenerative reactor cycles typically are either symmetric (same chemistry or reaction in both directions) or asymmetric (chemistry or reaction changes with step in cycle). Symmetric cycles are typically used for relatively mild exothermic chemistry, examples being regenerative thermal oxidation (“RTO”) and autothermal reforming (“ATR”). Asymmetric cycles are typically used to execute endothermic chemistry, and the desired endothermic chemistry is paired with a different chemistry that is exothermic (typically combustion) to provide heat of reaction for the endothermic reaction. Examples of asymmetric cycles are Wulff cracking, Pressure Swing Reforming, and other regenerative pyrolysis reactor processes. Regenerative pyrolysis reactors are generally known in the art as being capable of converting or cracking hydrocarbons. However, they have not achieved commercial or widespread use for hydrocarbon conversion, due at least in part to the fact that they have not scaled well to an economical size. This failure is in large part due to the inability of the equipment to adequately control and contend with the very high temperatures and the way that fuel and oxidant are combined during the regeneration or heating stage of the process. The high temperatures are difficult to position and contain for extended periods of time and lead to premature equipment failure. A solution was proposed in U.S. patent application Ser. No. 11/643,541 filed in the USPTO, on Dec. 21, 2006, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems, utilizing an inventive deferred combustion process.
As with steam crackers, regenerative pyrolysis reactors also are well suited for volatized or volatizable feedstocks that are substantially free of nonvolatile components, such as metals and other residual or nonvolatizable components, which would otherwise lay down and build up in the reactor as ash. Pyrolysis reactors typically operate at higher temperatures than steam crackers. Nonvolatiles may be defined broadly to mean any resid, metal, mineral, ash-forming, asphaltenic, tar, coke, or other component or contaminant within the feedstock that will not vaporize below a selected boiling point or temperature and which, during or after pyrolysis, may leave an undesirable residue or ash within the reactor system. The nonvolatile components of most concern are those that deposit as ash within the reactor system and cannot be easily removed by regeneration. Many hydrocarbon coke components may be merely burned out of the bed at the high temperature typically used in pyrolysis reactor systems and thus tend to be of less concern than some other residual components. Some nonvolatile feed components, such as metals and/or minerals, may leave an ash component behind that even at the high regeneration temperatures is difficult to remove from a reactor.
Typically, regenerative reactors include a reactor bed or zone, typically comprising some type of refractory material, where the reaction takes place within the reactor system. Conventional regenerative reactors typically deliver a stream of fuel, oxidant, or a supplemental amount of one of these reactants, directly to a location somewhere within the flow path of the reactor bed. The delivered reactants then are caused to exothermically react therein and heat the reactor media or bed. Thereafter, the reacted reactants are exhausted and a pyrolysis feedstock, such as a hydrocarbon feed stream, preferably vaporized, is introduced into the heated region of the reactor media or bed, and exposed to the heated media to cause heating and pyrolysis of the reactor feedstock into a pyrolyzed reactor feed. The pyrolyzed reactor feed is then removed from the reaction area of the reactor and quenched or cooled, such as in a quench region of the reactor system, to halt the pyrolysis reaction and yield a pyrolysis product.
However, as with steam cracking, economics may favor using lower cost feedstocks such as, by way of non-limiting examples, crude oil, heavy distillate cuts, contaminated naphthas and condensates, and atmospheric resids, as feedstocks for regenerative pyrolysis reactors. Unfortunately, these economically favored feedstocks typically contain undesirable amounts of nonvolatile components and have heretofore been unacceptable as regenerative reactor feedstocks. The nonvolatiles lead to fouling of the reactor through deposition of materials such as ash, metals, and/or coke. Regenerative pyrolysis reactors do not have the flexibility to process such otherwise economically crack favorable feedstocks because, although coke can typically be burned off, deposits or buildup of ash and metals within the reactor cannot easily be burned or removed. The critical concentration of nonvolatiles within a particular feedstock may vary depending upon variables such as the intended process, feedstock conditions or type, reactor design, operating parameters, etc. Generally, nonvolatile concentrations (e.g., ash, metals, resids, etc.) in excess of 2 ppmw (ppm by weight) of the feed stream to the reactor will cause significant fouling in a pyrolysis reactor. Some economically desirable lower cost feeds may contain up to 10 percent by weight of nonvolatiles, while still other feeds may contain well in excess of 10 weight percent of nonvolatiles. Since nonvolatiles do not vaporize, but decompose to form ash, metals, tar, and/or coke when heated above about 600° F. (315° C.) (in an oxidizing environment), the nonvolatiles present in disadvantaged feedstocks lay down or build up as a foulant in the reaction section of pyrolysis reactors, which increases pressure drop through the reactor and leads to plugging and decreased efficiency. Generally, only low levels of nonvolatiles (e.g., <2 ppmw and preferably <1 ppmw) or more specifically low levels of ash (measured by ASTM D482-03 or ISO 6245:2001) can be tolerated in the reactor feeds. Nonvolatiles are generally determined in accordance with ASTM D6560.
Various techniques have been employed for treating petroleum hydrocarbon feeds for the removal of nonvolatiles contained therein to render cost advantaged feeds suitable for conventional steam cracker feeds. These processes tend to improve the quality of hydrocarbon feeds containing nonvolatiles for conventional steam cracking. However, in most instances the processes suffer from operating condition limitations, space limitations for retrofits, high capital costs, and high operating costs, due to the processing steps used, high capital expense of the required equipment, and/or unsatisfactory reduction limitations in the amount of nonvolatiles removed from the feeds. For example, it may be quite costly to equip each of the several steam cracking furnaces in a steam cracking complex with all of the equipment necessary to process the low cost feedstocks to provide an acceptable, nonvolatile-free feed into the cracking section of each steam cracker. Similar and even exaggerated problems exist for a regenerative pyrolysis reactor complex, due to their feed quality requirements and increased temperature severity.
The present invention provides a revolutionary process for improving the quality of nonvolatile-containing hydrocarbon feedstocks to render such feed suitable for use as a feedstream to a regenerative pyrolysis reactor system. The invention provides a commercially useful and cost effective technique for removing the ash-forming nonvolatiles from the feedstock before the feedstock undergoes pyrolysis in a regenerative pyrolysis reactor.